A unified laminate of layers of polymer to which the instant invention has been successfully applied is described in U.S. Pat. application Ser. No. 471,320 to Browall et al., filed May 20, 1974. The Browall et al. (now U.S. Pat. No. 3,874,986) application is assigned to the assignee of the instant invention and is incorporated by reference. The laminate in the Browall et al. application consists of a first layer of microporous substrate, a second layer of polyphenylene oxide (PPO)/organopolysiloxane-polycarbonate and a third layer of organopolysiloxane-polycarbonate copolymer disposed between the first and second layers. The second and third membrane laminate (layers) are ultrathin films such as are prepared by the invention in Ser. No. 536,650 to Ward.
In a typical membrane composite, such as that described in the Browall et al. application, the microporous sheet will be about 5 mils thick while the physical thickness of the laminate of ultrathin membranes does not exceed about 300 A (equivalent to about 11.8 .times. 10.sup.-.sup.4 mil).
It has been found in the pilot production of such membranes that, because clean room techniques and chemical filters are not effective to exclude from the manufacturing area particulate matter below about 3000 A in size, the deposition of such very fine particulate foreign matter under and/or between the ultrathin polymer membrane layers is unavoidable. Because such particulate matter depositing under the ultrathin layers can be as much as 10 times as large as the sum of the thicknesses of the ultrathin layers, such particles can puncture one or both of these ultrathin layers and result in pinhole-size breaches through the membrane gas separation system. The effectiveness of gas separation by such membrane structures is diminished by the presence of such breaches.
It is, therefore, necessary in the fabrication of such membrane composites to maintain a station at which such leaks may be located as by the use of a dye check and spot patching of major leak spots may be accomplished. However, minor smaller leaks cannot be efficiently and economically cured in this manner.
U.S. Pat. No. 3,325,330 to Robb teaches that by bringing two partially cured films of organopolysiloxane together with sufficient pressure to exclude air from between the laminated films, the defects in one film are unlikely to be aligned with defects in the second film. In the Robb process of making films, however, it is required that the films (each initially about 2 mils thick) be stretched thin and then be cured to the solid, elastic state. Composite films as thin as about 0.1 mil are prepared in this manner. However, there is no indication (or likelihood) that the presence of particulate matter 10 times as thick as the composite film thickness (i.e. ranging to 1 mil in thickness) could be tolerated within the laminate, the problem to which the instant invention addresses itself.
The need has, therefore, arisen for a method for economically and effectively sealing both major and minor leak spots in the manufacture of laminates of ultrathin films.
The term "basic ultrathin membrane composite" describes the laminate containing all the laminae required to prepare a composite membrane able to function for gas separation in an effective manner when free of pinholes and imperfections and mounted on adequate support structure.
The term "layer" as employed herein includes a unified lamina consisting of one or more ultrathin films of a given material (e.g. PPO, organopolysiloxane-polycarbonate copolymer) in which the films are strongly bonded together, but need not merge.